This application claims the priority of PCT/EP98/04399, filed Jul. 15, 1998 and German patent document 197 31 329.9, filed Jul. 22, 1997, the disclosures of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for determining the pressure and temperature of a gas in a hollow space, and in particular of a fuel/air mixture in the combustion chamber of an internal combustion engine.
Known devices for determining the pressure and temperature of a fuel/air mixture in a combustion chamber of an internal combustion engine include as independent sensor components, a pressure-detecting element for sensing the pressure and a separate separately from it, a temperature-detecting element for sensing the temperature. The determination of the pressure and temperature of the fuel/air mixture in the combustion chamber is useful for sensing or monitoring the processes inside the engine, in particular for determining the engine load.
Engine load can be determined relatively accurately in steady-state operation of an engine, by measuring the throttle angle, measuring the intake pipe pressure using an appropriately positioned pressure sensor together with a measurement the temperature in the intake pipe, or by measuring the air mass by means of a hot-film air mass flowmeter. Deviations occur, however, in non-steady-state operation due to filling effects of the intake pipe, back-flow effects of residual gas in the valve overlap phase and pulsations in the intake pipe.
The use of cylinder pressure sensors (pressure sensors which are positioned in the respective combustion chambers of the engine cylinder) allows the air mass in the combustion chamber to be determined after closing of the inlet valves, by evaluating the pressure variation in the compression phase, without the determination being influenced by non-steady state effects. For this purpose, the ideal gas equation is used to calculate the cylinder charging. The required absolute pressure value can be calculated on the basis of the relative pressure sensed by means of the cylinder pressure measurement at a defined crank angle position, by a thermodynamic Pischinger zero correction, or by an intake pipe pressure equalization shortly before the closing of the inlet valve. The volume of the combustion chamber can be calculated using the crank angle and engine-specific data. The gas constant, also entered in the ideal gas equation, is known and can be regarded as approximately constant during the compression phase. If the temperature in the combustion chamber is then also measured, the charging mass can be determined from the ideal gas equation.
German Patent Document DE 40 10 609 A1 discloses a device for determining the pressure and temperature of a fuel/air mixture in the combustion chamber of an internal combustion engine, which forms an integrated part of a spark plug. The spark plug has a center electrode arranged in a tubular insulator and has a hollow portion surrounding a thermocouple which senses the temperature of the front end of the center electrode and consequently the combustion chamber temperature. The spark plug also has a metallic casing with a retaining seat around an outer surface of the casing. An annular pressure sensor is arranged on the retaining seat in such a way that it senses the combustion chamber pressure in a cylinder head, and the pressure sensor is arranged as an intermediate layer between the retaining seat and the cylinder head when the spark plug has been securely connected to the cylinder head of the internal combustion engine.
As an alternative to the use of thermocouples of this and similar types, it is known to determine the temperature of a fuel/air mixture in the combustion chamber of an internal combustion engine by ultrasonic delay measurement. See the Laid-open Patent Application DE 44 42 078 A1 and the papers appearing in journals, J. C. Livengood et al., Ultrasonic Temperature Measurement in Internal Combustion Engine Chamber, The Journal of the Acoustical Society of America, Vol. 26, No. 5, September 1954, Page 824 and G. Hohenberg, Gastemperatur-Mexcex2verfahren durch Laufzeitmessung von Ultraschallimpulsenxe2x80x94ein neuer Weg zur Erfassung innermotorischer Vorgxc3xa4nge (gas-temperature measuring method by delay measurement of ultrasonic pulsesxe2x80x94a new way of sensing processes inside an engine), Automobil-Industrie 2/75, Page 25. The first-mentioned paper discloses a device for determining the pressure and temperature of a fuel/air mixture in the combustion chamber of an internal combustion engine which comprises not only an ultrasonic delay measuring device but also a separate, independent pressure-measuring element for sensing the combustion chamber pressure. The last-mentioned paper describes the principle of temperature determination from ultrasonic delay (transit time) measurements using the known relationship for the speed of sound in a gas, dependent only on the gas constant, the quotient of the thermal capacities at constant pressure and at constant volume as well as the temperature. The distance required for calculating the speed of sound from the measured delays can be determined from the engine-specific geometric data, for example by utilizing the crank angle reference. For the ultrasonic delay measuring device, this paper proposes accommodating the ultrasonic transducer in an associated housing and screwing it into the cylinder head like a spark plug.
International Patent Document WO 91/19966 describes a pressure and heat flow sensor for the combustion chamber of an internal combustion engine, in which a pressure sensor element (for example, a piezoelectric element) and a heat-flow sensor element, comprising two temperature sensors, are integrated in a common housing.
One object of the present invention is to provide a pressure and temperature sensor of the type described above which is comparatively compact.
Another object of the invention is to achieve a reliable determination of the pressure and temperature of a gas in a hollow space, particularly a fuel/air mixture in the combustion chamber of an internal combustion engine.
Yet another object of the invention is to provide an operating method which achieves such reliable results.
These and other objects and advantages are achieved by the measuring method and application according to the invention, which comprises a combined pressure-sensor and ultrasonic-transducer unit with a measuring element that serves both for pressure measurement and as an ultrasonic transducer element for temperature determination by ultrasonic delay measurement. By integrating the pressure and temperature measuring functions into a single measuring element, the device can be made very compact and can therefore be located without any problem, for example, in the combustion chamber of an internal combustion engine. There is no need to provide separate pressure and temperature sensors in order to determine the pressure and temperature of a gas in a hollow space of known dimensions. Moreover, by providing this device at the combustion chamber of an internal combustion engine, the air mass in the combustion chamber can be calculated, even in non-steady-state phases of engine operation, comparatively accurately, using the aid of the ideal gas equation and the measured values for combustion chamber pressure and temperature.
With the pressure-sensor and ultrasonic-transducer unit combined in an integrated component of a spark plug, apart from the installation space already provided for the spark plug, no further space is required for the provision of pressure and temperature sensor elements on the combustion chamber walls.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.